13.8 Introduction to mycorrhizas

The mycorrhizal mutualistic association between fungi and
the roots of plants has contributed, from the earliest times, to the evolution
of the Earth’s terrestrial ecosystem. In this relationship the roots of the
plant are infected by a fungus, but the rest of the fungal mycelium continues to
grow through the soil, digesting and absorbing nutrients and water and sharing
these with its plant host. This was discovered by a German botanist called
Albert Bernhard Frank in 1885. He began a study of the
possibility of cultivating truffle groves in Prussia but his study developed
into a revolutionary theory of tree nutrition via symbiosis between fungi and
tree roots in a compound structure he called a Wurzelpilze, or
root-fungus. In fact, Frank used the words Wurzelsymbiose,
Wurzelpilze and Mycorhiza in the titles of three successive papers
in 1885 (and mykorhiza and mycorrhiza in titles of two later
papers). The spelling has now been standardised to ‘mycorrhiza’,
but it still means fungus-root! Later studies have shown that just about all of
Frank’s interpretations were correct.

A mycorrhiza is a mutualistic (or symbiotic) interaction between a fungus and
the roots of a plant the plant benefits from increased nutrient uptake
(particularly phosphate) via the fungal mycelium, while the fungus is supplied
with photosynthetic sugars by its host. The arrangement seems to have evolved as
soon as plants first colonised the land some 450 to 600 million years ago. Today
about 6,000 species of fungi are known to form
mycorrhizas with something like 240,000 plant species. Overall, 95% of vascular
plants have mycorrhizas associated with their roots, in all habitats, including
deserts, lowland tropical rainforests, through high latitudes and altitudes, and
including aquatic ecosystems. The few non-mycorrhizal plant families, such as
rushes (Juncus), sedges (Carex, Kobesia), Campions
(Silene) and crop plants such as rapeseed (Brassica napus;
one particular group of cultivars of which is known as canola), tend to colonise
open habitats, where competition for nutrients is reduced, particularly in
habitats where phosphorus availability is likely to be adequate
(Brundrett, 2009; Varma et al., 2017). For more information and
illustration of mycorrhizas visit Mark Brundrett’s website at
http://mycorrhizas.info.

Mycorrhizal fungi also link plants together into communities that are more
resilient to stress and disturbance than single plants. Nutrients can be
transferred between two different plants which are connected to the same
mycorrhizal system; for example from a well-placed donor plant to a shaded
recipient plant. The mycorrhizal interconnections form a
network through which plant-to-plant, plant-to-fungus and fungus-to-plant
transfers of nutrients and signalling molecules can take place
(it's been called the 'Wood-Wide-Web'; check out this clip from the BBC World
Service programme How trees
secretly talk to each other). In nutrient-poor soils,
mycorrhizal fungi can provide nitrogen to their host plant that their mycelia
have obtained by saprotrophic digestion of nutrients in the soil. Van der
Heijden & Horton (2009) completed a comprehensive review of the importance of
mycorrhizal networks in natural ecosystems and state that ‘mycorrhizal networks
play a key role in plant communities by facilitating and influencing seedling
establishment, by altering plant-plant interactions and by supplying recycled
nutrients’.

For plants, the arrangement has become the rule rather than the exception,
conversely, although there are common and important examples of mycorrhizas from
all the major types of fungi, most fungi are not mycorrhizal. The mycorrhizal
fungi you are most likely to meet are the mushrooms common in wooded areas.
Names already mentioned, like Amanita and Boletus are
mycorrhizal partners with trees and other forest plants, as are chanterelles,
and truffles too, although truffles are not mushrooms, of course, but a
hypogeous fruit body of the Ascomycota.